1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device of an in-plane switching mode capable of improving a transmittance and a contrast ratio of the liquid crystal display device.
2. Discussion of the Related Art
A liquid crystal display device displays an image by adjusting a light transmittance of liquid crystals. The liquid crystal display device has various modes according to the arrangement of liquid crystal molecules. For example, the liquid crystal display device has a twisted nematic (TN) mode to control the liquid crystal directors by a vertical electric field and an in-plane switching mode to control the liquid crystal directors by a horizontal electric field.
The TN mode liquid crystal display device drives liquid crystals by a vertical electric field formed between pixel electrodes and common electrodes disposed on upper and lower substrates to face each other. The TN mode liquid crystal display device has an advantage of a large opening ratio, but has a disadvantage of a small viewing angle.
The in-plane switching mode liquid crystal display device includes a color filter array substrate and a thin film array substrate, which are disposed to face each other and have a liquid crystal layer disposed therebetween. The color filter array substrate includes black matrices for preventing light leakage and a color filter layer for imparting colors to the black matrices. The thin film transistor substrate includes gate lines and data lines defining unit pixels, thin film transistors formed at crossing positions of the gate lines and the data lines, and common electrodes and pixel electrodes formed parallel to each other to generate a horizontal electric field.
The in-plane switching mode liquid crystal display device has excellent viewing angle characteristics by a liquid crystal driving method using a horizontal electric field of common electrodes and pixel electrodes.
Referring to FIG. 1, a liquid crystal display device of a horizontal electric field application type includes a thin film transistor array 10 and a color filter array 15 facing each other while liquid crystals 9 are interposed therebetween. The color filter array 15 includes black matrices 3, color filters 5 and an overcoat layer 7 which are sequentially formed on an upper substrate 1. The black matrices 3 prevent light leakage and light interference between neighboring color filters. The color filters 5 include red (R), green (G) and blue (B) color filters such that light passing through the color filters 5 can express colors. The overcoat layer 7 serves to planarize the upper substrate 1 having the black matrices 3 and the color filters 5.
The thin film transistor array 10 includes gate lines 12 and data lines 14 which cross each other on a lower substrate 11 to define pixel regions, thin film transistors (TFTs) which are respectively connected to the gate lines 12 and the data lines 14, pixel electrodes 18 connected to the thin film transistors (TFTs), common electrodes 19 parallel to the pixel electrodes 18, and common lines 16 connected to the common electrodes 19.
The thin film transistors (TFTs) supply data signals from the data lines 14 to the pixel electrodes 18 in response to gate signals from the gate lines 12. An electric field is formed between the pixel electrodes 18 supplied with the data signals through the thin film transistors (TFTs) and the common electrodes 19 supplied with reference voltages through the common lines 16. The common electrodes 19 and the pixel electrodes 18 may be formed on different layers or the same layer. If the common electrodes 19 and the pixel electrodes 18 are formed on different layers, the common electrodes 19 are connected to the common lines 16 and supplied with reference voltages from the common lines 16. On the other hand, if the common electrodes 19 and the pixel electrodes 18 are formed on the same layer, the common electrodes 19 are connected to the common lines 16 through contact holes for exposing the common lines 16 and are supplied with reference voltages from the common lines 16.
If an electric field is formed between the pixel electrodes 18 and common lines 16, the liquid crystals 9 are rotated by the electric field. The rotation of the liquid crystals 9 is controlled according to the data signals.
An upper polarizing plate 2a and a lower polarizing plate 2b are attached to an outer surface of the upper substrate 1 and an outer surface of a lower substrate 11, respectively, to transmit light vibrating in a specific direction. Generally, a transmission axis x of the upper polarizing plate 2a and a transmission axis y of the lower polarizing plate 2b are arranged perpendicularly to each other.
An initial arrangement state of the transmission axes x and y of the polarizing plates 2a and 2b and the liquid crystals 9 is a factor for determining a display mode of the liquid crystal display device. Generally, the liquid crystal display device of an in-plane switching mode has a normally black mode which displays black on the screen if an electric field is not formed.
If an electric field is formed between the pixel electrodes 18 and the common electrodes 19 in the normally black mode, the liquid crystals 9 are arranged parallel to the electric field. In this case, the liquid crystals 9 should be driven by an angle larger than a specific angle from the initial arrangement state by the electric field to influence a transmittance. The light passing through the liquid crystals 9 arranged parallel to the electric field mainly passes through the lower polarizing plate 2b to represent gradation. However, since light passing through a portion of the liquid crystals 9 cannot pass through the lower polarizing plate 2b, it is impossible to influence a transmittance of the liquid crystal display device of an in-plane switching mode. The liquid crystals 9 incapable of influencing a transmittance are generated since an electric field is formed in an undesired direction in a certain region due to structural characteristics of the pixel electrodes 18, the common electrodes 19 and the common lines 16.
FIGS. 2A and 2B illustrate enlarged views of regions in which an electric field is formed in an undesired direction. Further, in FIGS. 2A and 2B, the direction of electric field is represented by bidirectional arrows 
Referring to FIGS. 2A and 2B, the pixel electrodes 18 and the common electrodes 19 include a number of fingers 18a and 19a formed parallel to each other in the pixel regions. Meanwhile, in order that signals are applied to pixel electrode fingers 18a and common electrode fingers 19a, it is necessary to provide connecting portions which are formed perpendicularly to the fingers 18a and 19a of the respective electrodes so as to connect the fingers 18a and 19a of the respective electrodes and supply signals thereto.
For example, as shown in FIG. 2A, the pixel electrodes 18 and the common electrodes 19 may be formed on the same layer. In this case, the pixel electrodes 18 include a number of pixel electrode fingers 18a and pixel electrode connecting portions 18b which are formed perpendicularly to the pixel electrode fingers 18a to connect the pixel electrode fingers 18a. Further, the common electrodes common electrodes 19 include a number of common electrode fingers 19a. 
As another example, as shown in FIG. 2B, the pixel electrodes 18 and the common electrodes 19 may be formed on different layers. In this case, the common electrodes 19 include a number of common electrode fingers 19a parallel to each other. The common electrode fingers 19a are connected to the common lines 16 formed perpendicularly to the common electrode fingers 19a and are supplied with reference voltages. Further, the pixel electrodes 18 include the pixel electrode fingers 18a parallel to the common electrode fingers 19a and pixel electrode connecting portions which are formed perpendicularly to the pixel electrode fingers 18a to connect the pixel electrode fingers 18a. 
When signals are supplied to the pixel electrodes 18 and the common electrodes 19 of the in-plane switching mode liquid crystal display device, the direction of electric field applied to most of the pixel regions faces the pixel electrode fingers 18a and the common electrode fingers 19a. However, in regions adjacent to the common lines 16 and the pixel electrode connecting portions 18b, the direction of electric field faces the common lines 16 and the pixel electrode connecting portions 18b. As for the reason, the common lines 16 and the pixel electrode connecting portions 18b are formed perpendicularly to the fingers 18a and 19a to distort the electric field formed between the common electrode fingers 19a and the pixel electrode fingers 18a. The distortion of electric field due to the common lines 16 and the pixel electrode connecting portions 18b causes a nonuniform direction of electric field in regions adjacent to the common lines 16 and the pixel electrode connecting portions 18b, that is, pixel region edge portions. In the regions having a nonuniform direction of electric field, there are generated an inefficient driving region A in which the liquid crystals are driven in a direction incapable of influencing a transmittance and a disclination region B in which the liquid crystals are driven in opposite directions not to transmit light at a boundary thereof.
The inefficient driving region A and the disclination region B deteriorate a transmittance and a contrast ratio of the liquid crystal display device, thereby reducing display quality of the in-plane switching mode liquid crystal display device.